The present invention relates to a method of controlling electric conduction through a thermal head, in particular, a method of controlling electric conduction through a thermal head which enable optimum electric conduction in an overall printing process executed on a card-like print medium.
To produce a card-like print medium, for example, a credit card, a cache card, a license card, or an ID card, a printing apparatus as a thermal printer is conventionally used which allows a thermal head to perform thermal transfer on the card via a thermal transfer film with an ink layer to print and record desired images, texts, or the like. Such an apparatus is disclosed in, for example, Japanese Patent No. 3366791.
Furthermore, for example, Japanese Patent Application Publication No. 7-214843 discloses a thermal printer that prints the entire surface of the card-like print medium, that is, performs what is called overall printing. Additionally, for example, Japanese Patent Application Publications No. 3-3092 and No. 2002-307735 disclose a technique of controlling the amount by which a sheet as a print target medium is conveyed or controllably varying electric conduction time for the thermal head, depending on an intended condition (for example, a temperature condition) regardless of whether or not the overall printing process needs to be executed on the card or sheet as a print target medium.
However, when the overall printing process is executed on the card or sheet as a print target medium with a finite length as described above, the following phenomenon may occur. The temperature of an environment in which the printer apparatus is used or an atmospheric temperature in an apparatus body may, for example, change an outer diameter dimension of a platen roller that conveys and supports the print target medium (card or sheet) during printing or vary a winding diameter dimension of an ink ribbon and thus the tension of the ink ribbon during printing and conveyance. Thus, disadvantageously, a print size may deviate from a preset design value (expansion or contraction may occur).
In case the entire surface of the print target medium is printed, if a printing continues in a condition that the print size varies as described above to displace the thermal head from an end of the print target medium (the thermal head is prevented from abutting against the print target medium via the ink ribbon), the ink ribbon may be disadvantageously broken by heating. To prevent the above-descried problems, electric conduction through the thermal head may be controlled to turn off at a predetermined timing regardless of whether or not unprocessed print data is present. However, in this case, the overall printing of the print target medium fails to be completed, resulting in a blank in a part of the print target medium.
In view of these circumstances, an object of the present invention is to provide a method of controlling electric conduction through a thermal head as well as a thermal printer which can prevent problems such as breakage of the ink ribbon to allow the overall printing to be executed on the print target medium.
Further objects and advantages of the invention will be apparent from the following description of the invention.